Analysts said the decision to sign the Additional Protocol of the nuclear Nonproliferation Treaty (NPT) in Vienna was the most "momentous" made by the Islamic Republic since agreeing to a 1988 cease-fire to end the Iran-Iraq War.

Not since then has Iran's regime faced such overwhelming pressure - this time unified and from abroad - and been forced to come together to end a crisis with an unpalatable decision.

"If America wanted to do anything hostile to us in the future, they can't say it is because of nuclear weapons," says Hussein Shariatmadari, editor of the conservative Kayhan newspaper. "The international consensus against Iran and the illusions [of Iran's nuclear plans] are now broken."

Iran's decision is the climax of a volatile debate between power centers inside Iran, prompted by revelations about Iran's nuclear programs in the past year.

Still, the West should "take this opening and look at the Iranian quest for nuclear weapons in a broader context" of a tough region, says Daryl Kimball, execu- tive director of the Arms Control Association. The US should "broadly examine a strategy that leads to an [improvement] between the US and Iran."

Indeed, adding its name to the protocol - which the US and most Western European countries have yet to sign on to - may be one part of a broader Iranian attempt to ease US-Iran tensions, says Abbas Maleki, the head of the International Institute for Caspian Studies in Tehran.

"If Washington is searching for some signals from Iran, these are signals," says Mr. Maleki, a former foreign minister. On points most often raised by the US - Iran's human rights record, ties to terrorism, regional meddling, and nuclear weapons - Tehran improved "on all of these in the last months, very delicately," he says.

Iran's acquiescence to inspections may not change America's dim view of the country's activities. In fact, top Iranian officials say the suspension of uranium enrichment is "temporary."

"This is a very important step forward," says Mr. Kimball. "But it's not the end of the road that is going to take Iran away from nuclear weapons."

Though Iran's nuclear saga is far from over, the signing was heralded by some as a necessary step, on that permits snap inspections with two hours notice.

President Mohammad Khatami said last week that "our religious faith should not allow us to seek nuclear weapons." But the final decision was based more on realpolitik, though many argue here that Iran's strategic situation - with nuclear Israel, Pakistan, India, and Russia nearby, and US troops deployed along borders with Iraq, Afghanistan, and in the Persian Gulf - demands a nuclear deterrent.

Iranian hard-liners had initially declared that Iran should withdraw from the NPT rather than permit "spies" from the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), into the country. But finally, all factions agreed to the deal. It grew out of deliberations by at least one high-ranking committee that spanned the conservative-reformist divide and reported to Supreme Leader Sayed Ali Khamenei, who made the final call.

Pressure on the regime surged in September, when the IAEA board of governors issued an ultimatum - spearheaded by the US - to come clean on its nuclear program, or face sanction from the UN Security Council.

Days before that deadline expired, the foreign ministers of three key European powers - Britain, France, and Germany - visited Tehran and brokered a deal on Oct. 21. Iran would sign the protocol and halt uranium enrichment in exchange for help with its peaceful nuclear-power program.

In November, the IAEA said it had found "no evidence" that Iran was pursuing the bomb, a point described as "impossible to believe" by a senior US official. But it also condemned Iran harshly for covering up sophisticated enrichment programs for nearly two decades.

Now, the protocol "represents a breakthrough for both the US and Europeans, who played good-cop, bad-cop roles, cajoling and persuading Iran," says Kimball.

One European diplomat says that respect was a key issue in brokering the deal. "The three foreign ministers said, 'We respect you, we will deal with you,'" the diplomat says. "It doesn't mean that if things break down, Iran will not think of the nuclear option again. But it gives an indication that, if they get what they want, they will not go for the bomb."

"It was a momentous decision ... that could only be done because of a crisis, with the threat of sanctions, or even possible military strikes," says another Western diplomat. "[The head of the Supreme National Security Council] Hassan Rohani was a key broker: He is close to the leader, the reformists can talk to him, and the right wing trusts him."

Some see Iran's decision as the "first big achievement" for "pragmatic conservatives that are coming through" and shedding the hard-line baggage that has helped stalemate politics in Iran, the diplomat adds.

Mr. Rohani is now whispered to be presidential material, after Mr. Khatami's mandate expires in 2005, in part because he walks a middle line. "Rohani is doing what reformers said we should do: Break the international consensus against us," says one Iranian analyst.

"Only [this decision] could create gaps between hard-line and moderate conservatives," he says. "It's not a victory for the reformists, because they don't get credit for it. But there has been a shift."

For many, that shift has helped pull Iran back from the brink of deeper isolation, and danger. "It is a courageous decision, considering regional realities," says Davoud Hermidas Bavand, a law professor at the Supreme National Defense University in Tehran. "For the past 200 years, Iran has been subject to outside invasion and aggression.... Iran is surrounded by nuclear powers. With that in mind, having access to nuclear power is key."